THE GIST

- The similarities could help to explain why cetaceans, humans, non-human primates and elephants all have such big brains along with complex cognition.

- A high metabolic rate and other changes to energy usage in the body likely preceded the evolution of big brains in animals.

A close look at the dolphin genome reveals striking similarities between dolphins and humans.

The new study, published in the latest Proceedings of the Royal Society B, suggests that certain genetic features have led to the convergent evolution of large brains and complex cognition in a handful of species, including dolphins and humans.

"It has long been realized that dolphins rank among the most intelligent mammals, and they can do many things that great apes can do such as mirror self-recognition, communication, mimicry, and cultural transmission," lead author Michael McGowen told Discovery News, adding that dolphin brains are also "distinct and different."

McGowen is a researcher in the Center for Molecular Medicine and Evolution at Detroit's Wayne State University School of Medicine. He and colleagues Lawrence Grossman and Derek Wildman compared approximately 10,000 protein coding genes culled from the dolphin genome with comparable genes from 9 other animals: a cow, horse, dog, mouse, human, elephant, opossum, platypus and chicken. Out of that group, cows are most closely related to dolphins. The two animals are separated by 70 million years of history, however.

Similarities immediately became evident between dolphins, humans and elephants. All are animals known for their big brains and intelligence.

First, the scientists determined that the big brained trio have genes supporting a slow molecular rate. McGowen explained that this feature "has been connected to mammals with similar life histories, such as species with large generation times, large parental investment, and small effective population size. It happens that many of these species also have large brains, such as apes, elephants and cetaceans."

The researchers also found that these brainy animals had an adaptive evolution of their nervous system genes, proving that quality and not just quantity is important. In other words for brain function, size isn't everything. McGowen said that, in the brain, "folding, number of synapses, ratio of white matter to gray matter," and other factors appear to be predictable measures of intelligence.

The scientists also identified molecular signatures of metabolic evolution. This leads to a chicken and egg-type question: Which came first, the big brain or the changes to metabolism?

McGowen believes the latter evolved first.

"The big brain needs fuel, so we would guess that the changes to metabolism enabled the evolution of a big brain," he explained. "It's interesting that we are seeing the same modifications to the same groups of genes in lineages with large brains—primates, cetaceans, elephants. These include metabolic genes that provide the fuel for a brain, seeing as nervous tissue requires a lot more energy than other cells."

How Stuff Works: Dolphins

Dolphins additionally were discovered to have genes involved in human intellectual disorders and sleep. The former strongly suggests that those same genes are tied to intelligence, and "could be involved in the amazing cognitive capacity of dolphins," McGowen said.

As for sleep, the scientists found that a particular gene shared with humans and involved in wakefulness is altered in dolphins.

"Dolphins have an unusual form of sleep, in which only one side of the brain goes to sleep at a time, and during this state they continue to swim and have some awareness," he said. "This is exciting that we found a gene that could be related to this unusual distinctive feature."

All of these findings could strengthen the claim that dolphins are the world's second most intelligent animals.

"If we use relative brain size as a metric of 'intelligence' then one would have to conclude that dolphins are second in intelligence to modern humans," Lori Marino, Lori Marino, a senior lecturer in neuroscience and behavioral biology at Emory University, told Discovery News.

The findings about elephants might also help to explain why these big-brained pachyderms sometimes outwit humans who are trying to study them. Elephants often figure out tasks quickly, ignore human rules, and then cheat to earn the maximum food rewards.